Predicting the fountain flow instability

Instabilities in polymer processing limit production rates and may influence to some degree the optical or mechanical properties of the final product. One prominent example is the fountain flow instability, which takes place during the mold filling stage of an injection molding process. Old car bumpers which show light and dark lines are a famous example of an injection molded part in which such an instability occurred.

It has been shown experimentally that these instabilities manifest themselves in the form of a periodic oscillation of the flow front stagnation point between the two walls of the mold channels. The onset of instability is determined by a critical Weissenberg number, at which perturbations to the steady flow are amplified rather than damped.

We use the finite element method (FEM) and a number of stabilization techniques for flows of convective (SUPG) and viscoelastic (DEVSS-G) nature to do a full non-linear, transient numerical simulation of a branched polymer melt (XPP constitutive model) in a channel. We investigate both the onset and behavior of the fountain flow instability, with the focus on distinguishing the mechanisms that drive the instability, and their physical interpretation. After validation of the simulations with experiments we hope to use this new understanding to postpone or even remove entirely the fountain flow instability in injection molding processes.